The Extensible Markup Language (XML) is the standard for data and documents that is finding wide acceptance in the computer industry. XML describes and provides structure to a body of data, such as a file or data packet, referred to herein as an XML entity. The XML standard provides for tags that delimit sections of an XML entity referred to as XML elements. Each XML element may contain one or more name-value pairs referred to as attributes.
By defining an element that contains attributes and descendant elements, the XML entity defines a hierarchical tree relationship between the element, its descendant elements, and its attribute. A set of elements that have such a hierarchical tree relationship is referred to herein as an XML document.
An important standard for XML is the XQuery 1.0 and XPath 2.0 Data Model. (see W3C Candidate Recommendation 8 Jun. 2006, which is incorporated herein by reference). One aspect of this model is that a XML data is represented by a hierarchy of nodes that reflects the hierarchical nature of the XML data. A hierarchy of nodes is composed of nodes at multiple levels. The nodes at each level are each linked to one or more nodes at a different level. Each node at a level below the top level is a child node of one or more of the parent nodes at the level above. Nodes at the same level are sibling nodes. In a tree hierarchy or node tree, each child node has only one parent node, but a parent node may have multiple child nodes. In a tree hierarchy, a node that has no parent node linked to it is the root node, and a node that has no child nodes linked to it is a leaf node. A tree hierarchy has a single root node.
In a node tree that represents an XML document, a node can correspond to an element, the child nodes of the node correspond to an attribute or another element contained in the element. The node may be associated with a name and value.